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Fossil Polygonal Periglacial Structures in Flanders (Belgium)

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Fossil Polygonal Periglacial Structures in Flanders (Belgium) Permafrost, Phillips, Springman & Arenson (eds) © 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7 Fossil polygonal periglacial structures in Flanders (Belgium) I. Heyse & G. Ghysels Department of Geography, Physical Geography, Ghent University, Ghent, Belgium ABSTRACT: A systematic survey of aerial photographs in Flanders reveals an ephemeral visible polygonal pat- tern on the dissected plateau between the Coastal Plain and the Flemish Valley. The paper describes the charac- teristics of this pattern and discusses the origin and the age of these phenomena. The study is based on a comparative study of the literature, the analysis of aerial photographs, coupled with intensive fieldwork based on excavations. The spatial distribution of the sites, the visibility and the spatial structure of the patterns are described. Based on air photos the borders of the polygonal patterns were reconstructed and trenches were excavated in order to find underlying field evidence. The pattern corresponds with the infillings of frost wedges and exca- vated structures show characteristics of composite and ice-wedge pseudomorphs. The structures suggest forma- tion in a continuous permafrost environment as a result of thermal contraction cracking. These features have been constrained to the period between the Elsterian and Weichelian, but are most probably of Late Pleistocene age. 1 INTRODUCTION 1.1 Definition of the problem In Flanders (Belgium), various aerial photographs suggest the presence of polygonal networks (Fig. 2a), such as on the plateau between the Coastal Plain and the Flemish Valley (Figs. 1a, b). In this paper the origin and the age of these phenomena are discussed. According to Washburn (1973), polygonal networks have a worldwide occurrence. The largest polygons however, are restricted to cold periglacial and warm Figure 1a. General localisation of the study area. semi-arid environments. All the polygonal networks visible in the present- day temperate regions of the mid-latitudes are consid- ered as imprints from differential crop ripening of an underlying network of fossil frost-wedges, developed under palaeo-periglacial conditions (Christensen 1974; Gozdzik 1994; Johnson 1990; Kozarski 1993, 1995; Morgan 1971, 1972; Romanovskij 1973; Svensson 1972, 1973, 1976, 1984, 1988, 1990; Vandenberghe 1992; Walters 1994; Washburn 1973). Until now no evidence for these phenomena has been found in northern Belgium despite their pres- ence in other European countries such as Poland and the U.K. This detailed study of networks is based on the aerial photograph archive of Prof. Dr. Bourgeois (RUG, Department of Archaeology and Old History of Europe) that was gradually set up with the assis- tance of J. Semey. The term “frost-wedge” is used for all wedge struc- tures that originate due to thermal contraction in a frozen soil (French 1996; Kolstrup & Mejdahl 1986). The distinction between a permanent frost-wedge, developed in permafrost and a seasonal frost-wedge (“soil-wedge”), developed in a seasonally frozen Figure 1b. Localisation of the sites where research was ground is critical (Romanovskij 1985). carried out. 395 According to the primary infilling, distinctions are The controlling factors influencing their visibility made between ice-wedges, filled with ice, a mineral- are the thickness of the surface Quaternary cover, the wedge, filled with mineral material, and a composite- soil moisture differences, the contrast in lithology wedge, that consists of a mixture of both ice and mineral between host and fill materials, the type of growth, the material. On the basis of the degree of deformation of fos- period of the year and the meteorological conditions sil frost-wedges, a distinction is made between a “cast”, before and during the survey of the aerial photographs. that bears some resemblance to the original form and a “pseudomorph” that bears little resemblance to the origi- 2.2 The spatial distribution nal form (French 1996, p. 244). All the sites where polygonal networks occur are situ- 1.2 Methodology ated on the plateau between the Coastal Plain and the Flemish Valley (Fig. 1b). They are mainly located on This paper presents an analysis of aerial photographs low-angled (Ͻ1°), NW – NE orientated slopes, some- coupled with intensive fieldwork, based on excavations. times on the interfluves and the rims of valleys. Firstly, all the oblique aerial photographs were geo- referenced with the help of the software “ILWIS” 2.3 The spatial structure (RUG, Department of Regional Geography, Prof. Dr. Antrop M.). Subsequently, the polygonal networks Based on the size, the geometry and the spatial were mapped cartographically with the help of the pattern of the polygonal structures 6 types have been software “Arc View” (Fig. 2b). The aim of the geo- metrical transformation was to obtain correctly orien- tated and full-sized polygonal networks. Polygonal patterns were reconstructed in the field and trenches were excavated in order to find underlying structures. The observations of the polygonal networks in the field were impeded by soil formation, homogenisation and a high groundwater level during wintertime. Special attention is paid to visible macroscopic structures, because they are of prime importance to understand the mechanism of infilling. A relative age for the features is obtained, based on the geomorphological position, the lithological con- Figure 2a. Polygonal networks visible in arable land on an texts and the relation to other sedimentary structures aerial photograph, site 1 (coordinates: X ϭ 83.35 km; of the polygonal patterns. Y ϭ 197.25 km; Lambert-72), Aalter, Oost-Vlaanderen, Belgium (Photo: Semey J., 07-08-1990, slidenumber 53 1.3 Study area 026, Department of Archaeology and Old History of Europe, RUG). Arrow: orientation (North). The study area is situated in the northern part of the plateau between the Coastal Plain and the Flemish Valley (Fig. 1). The rolling landscape slopes to the northeast, with mean heights of 20 m in the north and 50 m in the south. A scarp and vale structure reflects successive denuda- tion in the Tertiary clayley and sandy substratum, infilled and levelled to some degree by Quaternary deposits (Heyse 1998). 2 ANALYSIS OF THE AERIAL PHOTOGRAPHS 2.1 Detection of polygonal networks The polygonal networks are periodically visible in cultivated land (Fig. 2a). The outline of the polygons is explained by variation in the ripening of crops and as a consequence the features are therefore known as Figure 2b. Cartographic representation of the networks “crop marks” (Svensson 1982). visible in Figure 2a. 396 Table 1. Typology of polygonal networks in Flanders, with characteristics and site properties. Figure 3. Wide sandy wedge structure developed in a ter- tiary clayley glauconiferous sandy substratum, Trench B-BЈ (see Figure 2b and arrow in Figure 2a for localisation), site 1 (coordinates: X ϭ 83.35 km; Y ϭ 197.25 km; Lambert- 72), Aalter, Oost-Vlaanderen, Belgium. Length of the spade: 1.2m. See Table 2, site 1 for characteristics. distinguished (Table 1). All 6 types belong to the “large type” (Ͼ1 m) following Washburn’s (1973) classification. Figure 4. Drawing of the sand-wedge structure visible in According to various authors the networks could be Figure 3. Note the notch-like margins and large host inclu- developed by two manners: sions in the fill. • Owing to thermal contraction under cold periglacial conditions during the Pleistocene (Karte 1987; Pissart 1987; Vandenberghe & Pissart 1993). • As a result of desiccation under warm semi-arid con- ditions during the Pre-Quaternary period, analogous to those described by Neal et al. (1968, in Murton et al. 2000) in current warm semi-arid regions. 3 FIELD EVIDENCES Figure 5. Excavation A-B, site 15 (coordinates: X ϭ The polygonal crop marks are associated with sand- 80.00 km, Y ϭ 199.00 km; Lambert-72), Maria-Aalter, wedge structures (Figs 3, 4 and 6) in the subsoil. In out- Oost-Vlaanderen, Belgium. Sandy involutions developed in crops sand-wedge structures are visible with mean the tertiary green sandy glauconiferous substratum. Localisa- spacings of 3 to 10 m (Fig. 5). The term “sand-wedge” tion of Figure 5 (rectangle). See site 15 on Figure 1b for is used in a descriptive way, to indicate a sandy sedi- localisation of the excavation site. mentary structure. All the characteristics of the struc- tures are summarised in Table 2. The tops of the is variable, ranging from wedge-shaped to strongly structures are between 0.4 m and 0.75 m depth, the bases irregular, sack-shaped structures. between 1.2 m and 1.8 m. The high width-height ratio, All the structures are filled with a complex of sandy varying between 1.5 and 2.7 is particular. The shape material with scattered pebbles (Figs 3, 4 and 6). 397 Table 2. Characteristics of the sand-involutions. Site Type structure Height (cm)Width (cm) W/H Spatial Litho Chrono 1 Wedge 80 140 1.8 P s/Gs IV/III 4 Sack-shaped - C 60/90 240/115 2.7/1.92 P s/Gw IV/III 6 Deformed sack 115/75 100 2 P s/u IV/III 8 Sack-shaped 65 125Ͻ 1.9 P s/l IV/IV 15a Deformed sack - C 120 100 1.7 P* s/Gs Sst IV/III 15b Wide sack 100 90 0.9 P* s/Gs IV/III 15c Long-waved sack - C 115 600 5.2 P* s/Gs IV/III Legend: C: composite structure, W/H: width-height ratio, P: polygonal structure, *: sites with polygonal patterns in the neigh- bourhood, not observed at the site itself. Lithological units: s: sand, w: clayey sand, u: clay, l: loam, G: glauconite, Sst: sandstone fragments. Chrono: III: Tertiary, IV: Quaternary. Remark: structure 15c is an obliquely cut structure. fill with large host inclusions (Figs 3, 4 and 6), a con- cave downward curved lamination/inclusions (Fig. 6) and scattered pebbles (1–10 cm) (Figs.
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